The superior optical characteristics of polysiloxane-based silicon resins have attracted considerable attention in the field of optical and optoelectronic elements. The rubber elasticity of polydimethylsiloxane is essential for use in intraocular lenses, varifocal optical elements, and the like, where flexibility is required in addition to optical properties. Also, investigations have been conducted into using polysiloxanes in the manufacture of wave guides. Moldable polysiloxane-based composite optical materials made up of finely divided silica and polysiloxane resin are disclosed in JP-A-07-294701. These materials represent a type of polysiloxane resin that can be used for ordinary optical elements that require rigidity along with optical characteristics. One of the characteristics of polysiloxanes is their relatively low index of refraction as compared with hydrocarbon resins.
One advantage of a low index of refraction is the suppression of reflection at interfaces when light passes through such materials. High polymers having fluorine atoms in the molecule are known in the art to have a low index of refraction. Compared with ethylene-propylene copolymers, the index of tetrafluoroethylene-hexafluoropropylene copolymers is as low as approximately 0.1. Polysiloxanes obtained by bonding fluorohydrocarbon radicals in polysiloxane resins, and thus containing fluorohydrocarbon radicals, to improve the water repellency and oil repellency of polysiloxane resins have been disclosed in JP-A-55-48245 (1980) and U.S. Pat. No. 5,371,155 issued to Kobayashi on Dec. 6, 1994. However, such resins were developed without paying attention to their optical characteristics, and there have been various problems concerning their application in optical elements.
Transparent materials with a low index of refraction are applied as the outermost coating layer in optical elements to provide a reflection-preventing film. Magnesium fluoride, MgF.sub.2, has been widely used as a coating material for forming such reflection-preventing films. However, its use requires films to be formed on the surface of optical elements by means of sputtering, and uniform coating of large surfaces of organic materials has posed serious difficulties in terms of techniques and equipment.
Improving resin strength by adding micro particles of finely divided silica to polysiloxane resin is a technique known in the art. However, in many cases, this technique gives optically opaque compositions. For example, in JP-A-07-294701 (1995), the resins disclosed are optically opaque when fluorohydrocarbon radicals are introduced.
One object of this invention is to provide an optical polysiloxane resin suitable for optical elements. A further object of this invention is to provide an optical polysiloxane resin exhibiting a superior light transmissivity and a high surface hardness based on the use of a polysiloxane resin, having radicals selected from the group consisting of hydrocarbon radicals, fluorohydrocarbon radicals, and combinations thereof.